The microelectronic industry has made remarkable progress with the development of integrated circuit (IC) technology, fueled by Moore's law (Intel), which is the doubling of the number of transistors on a single chip every 2-3 years. This enables faster and more sophisticated semiconductor circuits. The exponential growth of this technology depends on the fabrication of smaller feature sizes down to the nanometer scale, forecasted as the 22 nm half pitch node.
Although EUV lithography at 13.5 nm wavelength has emerged as a promising candidate to meet the resolution requirements of the microelectronic industry roadmap, the development of advanced photoresist materials with all of the required imaging properties remains challenging and is one of the major subjects of current nanolithography research. The requirements for such photoresists include high photospeed, high resolution, low line edge roughness, low outgassing, low pattern cross-sectional aspect ratio and profile, high pattern transfer and etch resistance, low defect density, and high reproducibility. Among the foregoing, high photospeed, high resolution and low line edge/line width roughness (LER/LWR) are the most critical requirements. The design of novel resist materials that can achieve all three characteristics is the key for the progression of Moore's law via the continued success of high resolution nanopatterning in integrated circuit manufacturing.
Two primary types of resist polymers that have been investigated are chain-scission resists and pendant chemically amplified (CA) resists. Upon irradiation of a chain-scission resist film, the molecular weights of the polymers in the exposed regions are decreased via chain scission reactions arising from the irradiation. As a result, solubility differentiation is achieved between the exposed and the unexposed regions. Chemically amplified resists, however, achieve solubility differentiation based on an acid-catalyzed deprotection reaction which changes the polarity of the polymer in the exposed region. A typical CA resist formula consists of a matrix polymer and a photoacid generator (PAG). Upon irradiation with an electron beam or extreme UV radiation, the PAG generates a strong acid that catalyzes the deprotection reaction.
Several classes of PAGs have been used in CA resists. These PAGs, however, are almost exclusively used in their small molecule forms, and small molecule PAGs typically exhibit limited compatibility with the resist polymer matrix. As a result, problems such as phase separation, non-uniform acid distribution, and non-uniform acid migration occurring during temperature fluctuations (e.g. variation in baking processing) may arise. Such limitations frequently lead to an undesirable, premature and non-uniform deprotection reaction in the CA resist film.